Many different methods for the preparation of epoxides have been developed. Commercially, propylene oxide is produced by the chlorohydrin process or hydroperoxidation (see, e.g., U.S. Pat. Nos. 3,351,635 and 4,367,342; EP 0 345 856). Unfortunately, both processes have disadvantages. The chlorohydrin process suffers from the production of a dilute salt stream. The hydroperoxidation process, in which propylene is oxidized with an organic hydroperoxide such as ethylbenzene hydroperoxide or tert-butyl hydroperoxide, produces organic co-products such as t-butyl alcohol or styrene, whose value must be captured in the market place. Ethylene oxide is commercially produced by the direct oxidation of ethylene with oxygen over a silver catalyst. Unfortunately, efforts to epoxidize higher olefins (olefins containing three or more carbons) such as propylene with oxygen in the presence of a silver catalyst have failed to produce a commercial process (see, e.g., U.S. Pat. Nos. 5,856,534, 5,780,657 and 4,994,589).
Recent efforts have focused on the direct epoxidation of higher olefins with oxygen and hydrogen. For example, the reaction may be performed in the presence of a catalyst comprising gold and a titanium-containing support (see, e.g., U.S. Pat. Nos. 5,623,090, 6,362,349, and 6,646,142), or a catalyst containing palladium on a titanium zeolite (see, e.g., JP 4-352771).
Mixed catalyst systems for olefin epoxidation with hydrogen and oxygen have also been disclosed. For example, Example 13 of JP 4-352771 describes the use of a mixture of titanosilicate and Pd-on-carbon for propylene epoxidation. U.S. Pat. No. 6,008,388 describes a catalyst comprising a noble metal and a titanium or vanadium zeolite, but additionally teaches that the Pd can be incorporated into a support before mixing with the zeolite. The catalyst supports disclosed include silica, alumina, and activated carbon. U.S. Pat. No. 6,498,259 discloses the epoxidation of an olefin with hydrogen and oxygen in a solvent containing a buffer in the presence of a catalyst mixture containing a titanium zeolite and a noble metal catalyst. It also discloses that organic polymer resins such as polystyrene, styrene-divinylbenzene copolymer, crosslinked polyethyleneimines, and polybenzimidazole may be used as supports for the noble metal catalyst.
Recently, a technique called “microencapsulation” was used to prepare catalysts with improved properties (see Chem. Commun. (2003) 449 and references cited therein; Angew. Chem., Int. Ed. 40 (2001) 3469; J. Am. Chem. Soc. 120 (1998) 2985; U.S. Appl. Pub. Nos. 2005/0201925, 2005/0202957, and 2005/0203304). EP 0 498 166 Al discloses an alumina-supported Pd catalyst impregnated with 4-bromostyrene or styrene that is subsequently polymerized (see Example 7 and Comparative Example 8). The poly(4-bromostyrene)-coated catalyst is active in generating hydrogen peroxide from hydrogen and oxygen. U.S. Appl. Pub. No. 2004/0184983 describes a catalyst consisting of: (a) one or more metals of the platinum group as active components; (b) one or more polyolefins; and (c) a carrier. The polyolefin is dissolved in a solvent and the resulting solution is used to impregnate the carrier or the catalyst.